Road lighting installations, or those intended for big industrial and sporting areas involve the need of being able to change the luminous intensity as a function of the real use requirements, in order to be able to effect a remarkable saving of electric energy and lengthen the life of the lamps.
Indeed it is recognized that in public lighting installations the luminous intensity required and absolutely necessary during the first night-hours is excessive after a certain time, and if not reduced, becomes an unnecessary waste of energy. Moreover, during the same night-hours, due to the stay in producing activities, the mains voltage tends to rise above the voltage rating, thus reducing the life of the lamps.
The known systems for reducing the luminous intensity of public lighting installations are basically grounded on the use of an autotransformer for changing the voltage on the load (lamps). In the first known solution, the primary winding of the autotransformer comprises some parallel shunts by means of which, through properly connected switches, the power supplied to load is discretely changed with coarse values of the changes. The drawback of this solution is therefore that the lamps are subject to a series of stresses, jeopardizing the life thereof. The second known solution provides for continuously changing the power supplied to the lamps, however using devices with moving, wiper contact arrangements (for example, a VARIAC). Thus, they are inclined to wear, seizing and other drawbacks typical of moving parts, including the requisite of periodical controls in order to check the wear of sliding parts, inclined to wear, and the proper alignment of moving parts.